This application relates to vertical axis wind turbines and methods of operating vertical axis wind turbines.
Wind energy is a fast growing renewable resource that will play a factor in reducing the world's reliance on fossil fuels. The most common wind turbine today is the horizontal axis wind turbine (HAWT). This type of turbine has the highest coefficient of performance currently available, but it is expensive and requires frequent maintenance. This results in considerable down time because the power generator and other electrical equipment is located at the top of the turbine.
Darrieus turbines are a type of vertical axis wind turbine (VAWT) that can become competitive with the HAWT. Benefits of the Darrieus turbine are that it is generally much cheaper than the HAWT. It is also omnidirectional, quiet, and easier to maintain than the HAWT. There is a need for a wind powered turbine that can be maintained at a low cost while producing more power than a traditional Darrieus turbine.
The wind industry is growing on a global and national level. The United States Department of Energy (DOE) aims for 20% of the nation's electricity to be produced from wind by 2030. The DOE also states that “greater use of the nation's abundant wind resources for electric power generation will help the nation reduce emissions of greenhouse gases and other air pollutants, diversify its energy supply, provide cost-competitive electricity to key regions across the country, and reduce water usage for power generation.” Wind energy is a fast growing renewable resource that will play a factor in reducing the world's reliance on fossil fuels.
Location is important when deciding where to install wind turbines. Wind availability in an area is defined by seven wind power classes in the United States. Wind farms are only built in areas with class 3 or higher wind speeds because slower speeds do not produce enough electricity to make commercial turbines economically viable. Residential and private use, however, can go as low as class 2 wind speeds because it is a smaller application.
The most common wind turbine today is the horizontal axis wind turbine (HAWT). This type of turbine has the highest coefficient of performance currently available and operates by producing lift. Lift is a force that is perpendicular to the fluid motion on the airfoil. The wind comes from one direction, but the velocity seen from a moving airfoil, or relative velocity, is what will produce lift in a turbine blade. In order for the turbine blade to turn faster than the wind lift force must exceed the drag force. Drag force is parallel to the relative velocity and is present throughout the whole circle of rotation. Lift force, however, is only present when there is a low pressure zone on one side of the airfoil. This means that there are zones in a full revolution where no lift is produced.
HAWTs used in a utility-scale operation range in size from 100 kilowatts to a few megawatts. Wind turbines that produce less than 100 kilowatts are often used for residential applications. FIG. 2 shows a wind farm containing an array of horizontal axis wind turbines. HAWTs may be used predominately in industry, but they are not without flaws.
The main problems with the HAWT are the price and the fact that the power generator and other electrical equipment are located at the middle of the turbine, generally at the top of a tower. This makes maintenance difficult, so the operation and maintenance costs of new turbines are 20-25% of the annual profit. Turbine maintenance can take 1 to 7 days of down time for each repair depending on the part that needs to be replaced. In addition to downtime required for maintenance, the structure that supports the turbine needs to be sturdy enough to hold up the heavy generator equipment as well. For example, a structure of a small turbine that is only eighty feet tall accounts for approximately 30 percent of the total system cost. The total cost for commercial HAWT can range from $45 million per unit.
Other issues with the HAWT include vibrations and location. People do not want to live near HAWTs because the turbine blades create vibrations as they sweep across the support column, causing harmonic interference. Zoning laws prohibit the building of HAWTs near residential areas for this reason.
Another type of wind turbine is the vertical axis wind turbine (VAWT). There are several varieties of VAWTs; some operate on drag forces while others operate on lift. All VAWTs rotate about their central axis, so each blade is always the same distance away from the support column. This means that these turbines are quieter than the HAWTs because the support column is equidistant from each turbine blade, which drastically reduces the vibrations caused by the blade passing by the column. The power generator and electronics for VAWTs are located at the base of the turbine, which provide easy access for maintenance. VAWTs are also omnidirectional, which means that the direction the wind is coming from does not matter.
The Darrieus turbine is a VAWT that operates by producing lift. This gives it the ability to compete with the HAWT because they both function by using the same principals. There are three main shapes of Darrieus turbines; the eggbeater shape, the H rotor, and the helical Darrieus. The coefficient of performance for the three types of Darrieus is very similar. Generally, Darrieus turbines are significantly cheaper than HAWTs, which is the primary reason they are considered over the HAWT. Like all VAWTs, they are also omnidirectional, quiet, and easier to maintain than the HAWT.
Two main drawbacks of the Darrieus turbine are that it may require a motor or other system to spin at low speeds and is not as efficient as the HAWT. The cut-in speed of a Darrieus turbine is the speed at which the turbine starts to generate positive power. Up until the cut-in speed the turbine consumes power by operating the motor that spins the turbine.